Human hearing is generally considered to be in the range of 20 Hz to 20 kHz, with greatest sensitivity to sounds in the range of 1 kHz to 4 kHz, and with high frequency hearing significantly deteriorating in many older people. In the hearing aid industry, the top of the frequency range of amplified sounds is often around 8 kHz.
One well known problem with hearing aids is their tendency to generate feedback, where the sound output by the receiver in the hearing aid travels back to the microphone in an acoustic feedback loop, to then be reamplified in the next cycle through the hearing aid. With repeated amplification by multiple cycles through the hearing aid, feedback can produce loud and annoying whistles, buzzes and pops in the sound output, which can significantly reduce the quality of the user experience. Moreover, the onset of feedback problems is difficult to predict and depends heavily on the specific acoustic conditions, both in ambient sounds received by the microphone at a particular time and location and in physical acoustic feedback path changes near the ear (such as moving a hand or placing a telephone near the ear). The likelihood of feedback increases with the amplification gain, but large amplification gains are frequently desired to make up for the user's hearing loss, and hearing aid designers must often limit the overall gain to avoid feedback. In hearing aids that use digital signal processors (DSPs), there is commonly an algorithm applied to attempt to cancel or reduce the incidence of feedback.
The problem of feedback cancellation can be particularly difficult at high frequencies, such as in the 4 to 8 kHz range. When amplifying speech, spoken sounds such as “s”, “sh”, “t” and “k” generally include significant high frequency components which extend broadly through a number of frequency bands in this 4 to 8 kHz range. At the same time, acoustic path changes can result in large phase changes at such high frequencies. If the acoustic path changes occur rapidly, then it is difficult for a DSP feedback canceller to track the changes. Some DSP hearing aids have used broad-based frequency shifting in an attempt to reduce feedback problems, but existing solutions are unsatisfactory. Better methods of avoiding high frequency feedback in hearing aids are needed, particularly for use in amplifying speech and with those having degraded hearing in the higher frequency ranges.